JP2016144243A - Over current detection circuit for switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an over current detection circuit capable of detecting an overcurrent state of an output current even without interposing a resistor for detection in a path where the output current flows.SOLUTION: An overcurrent detection circuit 2 is configured to detect an overcurrent state of an output current Iin a flyback switching power supply device 1 which outputs an output voltage Vobtained by a first rectifying/smoothing part 3 for rectifying and smoothing an induction voltage in a secondary coil Tof a transformer T. The overcurrent detection circuit comprises: a second rectifying/smoothing part 4 that is provided separately from the first rectifying/smoothing part 3, for rectifying and smoothing the induction voltage; and a detection part 5 for detecting the overcurrent state of the output current Iif a voltage Vfor detection that is obtained by the second rectifying/smoothing part 4 is higher than a predetermined threshold voltage.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an overcurrent detection circuit that detects that an output current of a switching power supply device is in an overcurrent state.

  As an overcurrent detection circuit that detects that the output current of the switching power supply device is in an overcurrent state, for example, the one described in Patent Document 1 is known. This overcurrent detection circuit amplifies the voltage across the detection resistor interposed in the output current path, and detects the overcurrent state based on the amount of the amplified voltage. According to this overcurrent detection circuit, since the resistance value of the detection resistor can be made smaller than when amplification is not performed, power loss due to the detection resistor can be reduced.

JP-A-5-236642

  However, in this conventional overcurrent detection circuit, if the resistance value of the detection resistor is reduced, the ratio of noise to the signal indicating the amount of output current increases, resulting in trouble in detection. There is a problem that the resistance value cannot be reduced so much that the power loss cannot be sufficiently reduced.

  The present invention has been made in view of the above circumstances, and it is an object of the present invention to detect an overcurrent state of an output current without providing a detection resistor in the path through which the output current flows. It is to provide a current detection circuit.

  In order to solve the above problems, an overcurrent detection circuit according to the present invention is a flyback switching power supply that outputs a voltage obtained by a first rectifying / smoothing unit that rectifies and smoothes an induced voltage in a secondary winding of a transformer. In the device, an overcurrent detection circuit for detecting that the current output from the first rectifying and smoothing unit is in an overcurrent state, the rectifying and smoothing of the induced voltage provided separately from the first rectifying and smoothing unit And detecting that the current output from the first rectifying / smoothing unit is in an overcurrent state when the voltage obtained by the second rectifying / smoothing unit and the voltage obtained by the second rectifying / smoothing unit are larger than a predetermined threshold voltage. And a detector.

In the flyback type switching power supply device, when the current (output current I _OUT ) output from the first rectifying / smoothing unit increases, the surge voltage due to the leakage inductance of the transformer increases, but the smoothing capacitor constituting the first rectifying / smoothing unit Has a sufficient discharge amount, and the voltage (output voltage V _OUT ) output from the first rectifying / smoothing unit is maintained at a constant voltage by feedback control. Therefore, even if the surge voltage increases, the output voltage _VOUT does not rise. On the other hand, the feedback control as described above is not performed on the voltage (detection voltage V _D ) output from the second rectifying / smoothing unit provided for detecting the overcurrent state. Further, the discharge amount of the smoothing capacitor constituting the second rectifying / smoothing unit is small. Therefore, when the smoothing capacitor constituting the second rectifying / smoothing unit is charged with a high voltage affected by the surge voltage, the detection voltage V _D increases as the surge voltage increases. That is, the detection voltage V _D increases as the output current I _OUT increases.

According to the overcurrent detection circuit, by comparing the predetermined threshold voltage and the detection voltage V _D which increases with increase in the output current I _OUT, that the output current I _OUT is in the overcurrent state Can be detected.

As a specific configuration of the above-described overcurrent detection circuit, for example, the detection unit has at least two voltage dividing resistors that divide the voltage obtained by the second rectifying and smoothing unit, and a voltage obtained by the voltage dividing resistor in advance. A configuration is conceivable that includes a shunt regulator that becomes conductive when the set voltage is higher than a predetermined set voltage, and a detection resistor connected in series to the shunt regulator. In this case, it is possible to transmit to the primary side that the output current _IOUT is in an overcurrent state by further providing a photocoupler that transmits a signal related to the voltage across the detection resistor to the primary side.

Note that a current flows through the detection resistor only when the output current _IOUT is in an overcurrent state. On the other hand, an output current always flows through the detection resistor in the conventional overcurrent detection circuit. The detection resistor in the overcurrent detection circuit according to the present invention is decisively different in this respect from the detection resistor in the conventional overcurrent detection circuit.

  According to the present invention, it is possible to provide an overcurrent detection circuit capable of detecting an overcurrent state of an output current without providing a detection resistor in a path through which the output current flows.

It is a circuit diagram of a flyback type switching power supply device provided with an overcurrent detection circuit according to the present invention. It is a graph which shows the relationship between an output current and the voltage for a detection.

  Embodiments of a switching power supply device including an overcurrent detection circuit according to the present invention will be described below with reference to the accompanying drawings.

The switching power supply device 1 according to the present embodiment converts the input voltage _VIN supplied from the DC power supply 10 into a DC voltage to be supplied to the load 11 (output voltage V _OUT , 15.5 [V] in the present embodiment). It is a flyback type switching power supply device. As shown in FIG. 1, a switching power supply device 1 includes a transformer T, a switching element Q, a first rectifying / smoothing unit 3 and a control unit 6, and a second rectifying / smoothing unit 4 constituting an overcurrent detection circuit 2 according to the present invention. And a detection unit 5 and a photocoupler PC.

First rectifying smoothing unit 3 includes a rectifying diode _{D 1} and a smoothing capacitor _{C 1.} First rectifying smoothing unit 3 rectifies and smoothes the voltage induced in the secondary winding T ₂ of the transformer T when the conduction states of the switching elements Q that are connected in series to the primary winding T ₁ of the transformer T has changed The output voltage V _OUT thus obtained is output toward the load 11.

The control unit 6 feeds back the conduction state of the switching element Q so that the voltage value of the output voltage _VOUT detected by a voltage detection unit (not shown) becomes a predetermined value or falls within a predetermined range. Control.

Second rectifying smoothing unit 4 constituting the overcurrent detection circuit 2 includes a rectifying diode D ₂ and a smoothing capacitor C _2. Like the first rectification smoothing unit 3, the second rectification smoothing unit 4, the voltage induced in the secondary winding T ₂ rectifies and smoothes the transformer T, and outputs the detection voltage V _D obtained thereby.

The detection unit 5 includes voltage dividing resistors R ₁ and R ₂ that divide the detection voltage V _D , a shunt regulator REG to which a voltage obtained by the voltage dividing resistors R ₁ and R ₂ is input as a reference voltage, and a shunt regulator and a detection resistor _{R 4} and the resistor _{R 3} connected in series REG. The shunt regulator REG becomes conductive when the reference voltage is larger than a predetermined set voltage. When the shunt regulator REG becomes conductive, a current flows through the path of the second rectifying / smoothing unit 4 → the resistor R ₃ → the detection resistor R ₄ → the shunt regulator REG, and a voltage is generated at both ends of the detection resistor R ₄ .

In this embodiment, the resistance values of the voltage dividing resistors R ₁ and R ₂ , the detecting resistor R ₄ and the resistor R ₃ are 82 [kΩ], 15 [kΩ], 1 [kΩ], and 1 [kΩ]. Further, as described above, the output voltage _VOUT is maintained at 15.5 [V]. For this reason, a current of about 15.5 [V] / 97 [kΩ] = 160 [μA] always flows through the voltage dividing resistors R ₁ and R ₂ , which is detected by a conventional overcurrent detection circuit. Compared to the output current flowing through the resistor, it is quite small.

Further, in this embodiment, the shunt regulator REG is turned, along with the voltage of 1.2 [V] which is limited by the forward voltage V _F of the photocoupler PC is applied to the detecting resistor R _4, the resistor R ₃ is a voltage (15.5 [V] −1.2 [V] −2.45 [V] excluding 2.45 [V] of the forward voltage V _F and the minimum cathode voltage V _KA of the shunt regulator REG. V] = 11.85 [V]) is applied. Therefore, the detection resistor R ₄ and the resistor R ₃ have a resistance of about 1.2 [V] / 1 [kΩ] = 1.2 [mA], respectively, 11.85 [V] / 1 [kΩ] = 11. .85 [mA] degree of relatively large current flows, the shunt regulator REG is in a normal state where a non-conductive state, current does not flow through the detection resistor R ₄ and the resistor R _3.

The photocoupler PC includes a photodiode connected to the detection resistor R ₄ and a phototransistor connected to the control unit 6. When the voltage across the sense resistor R ₄ occurs, the photocoupler PC is optically transmitted to the control unit 6 provided a signal indicating that the voltage is generated across the sense resistor R ₄ to the primary side . As will be described in detail later, the generation of a voltage across the detection resistor R ₄ means that the output current I _OUT is in an overcurrent state.

When it is transmitted that the output current I _OUT is in an overcurrent state, the control unit 6 stops controlling the switching element Q. As a result, the output voltage _VOUT decreases and the overcurrent state is eliminated.

In the switching power supply device 1, the output current I _OUT increases, but a surge voltage due to the leakage inductance of the transformer T is increased, the smoothing capacitor C ₁ which constitutes a first rectification smoothing unit 3 has a sufficient amount of discharge, moreover Since the voltage (output voltage V _OUT ) output from the first rectifying and smoothing unit 3 is maintained at a constant voltage by feedback control, the output voltage _VOUT does not increase even if the surge voltage increases. On the other hand, the feedback control as described above is not performed for the detection voltage V _D. The discharge amount of the smoothing capacitor C ₂ constituting the second rectification smoothing unit 4 is slight. Accordingly, the smoothing capacitor C ₂ is charged at a high voltage under the influence of surge voltage, the detection voltage V _D increases with increasing surge voltage. That is, the detection voltage V _D increases as the output current I _OUT increases. In this embodiment, for example, the smoothing capacitor C ₁ constituting the first rectifying / smoothing unit 3 has a rated voltage / rated capacitance of 35 [V] / 470 [μF] and the smoothing capacitor constituting the second rectifying / smoothing unit 4. The rated voltage / rated capacitance of C ₂ can be set to 50 [V] / 1 [μF].

When the detection voltage V _D increases with an increase in the output current I _OUT, also increases the reference voltage input to the shunt regulator REG. When the reference voltage becomes larger than a predetermined set voltage, the shunt regulator REG becomes conductive.

FIG. 2 shows the relationship between the output current I _OUT and the detection voltage V _D in this embodiment. As an example, when 45 [mA] is the boundary between the overcurrent state and the normal state, the set voltage of the shunt regulator REG is 2.45 [V] (= 15.85 [V] × 15 [kΩ] / ( 82 [kΩ] +15 [kΩ])), it can be detected that the detection voltage V _D is 15.85 [V] or more, that is, the output current I _OUT is in an overcurrent state. .

Although the embodiment of the overcurrent detection circuit 2 according to the present invention has been described above, the configuration of the present invention is not limited to the above embodiment. For example, the detection unit 5 is capable of detecting any detection voltage V _D obtained by the second rectification smoothing unit 4 that is larger than a predetermined threshold voltage (15.85 [V] in the embodiment). The circuit can be changed.

1 switching power supply unit 2 overcurrent detection circuit 3 first rectification smoothing unit 4 second rectifying smoothing unit 5 detecting unit 6 control unit 10 DC power supply 11 load _C 1, _{C 2} smoothing capacitor _D 1, _{D 2} rectifier diode PC photocoupler Q Switching element R ₁ , R ₂ Voltage dividing resistor R ₄ Detection resistor T Transformer T ₁ Primary winding T ₂ Secondary winding

Claims (3)

In a flyback type switching power supply that outputs a voltage obtained by a first rectifying / smoothing unit that rectifies and smoothes an induced voltage in a secondary winding of a transformer, the current output from the first rectifying / smoothing unit is in an overcurrent state. An overcurrent detection circuit for detecting that
A second rectifying / smoothing unit provided separately from the first rectifying / smoothing unit for rectifying and smoothing the induced voltage;
A detection unit that detects that the current output from the first rectification smoothing unit is in an overcurrent state when the voltage obtained by the second rectification smoothing unit is greater than a predetermined threshold voltage;
An overcurrent detection circuit comprising: The detector is
At least two voltage dividing resistors for dividing the voltage obtained by the second rectifying and smoothing unit;
A shunt regulator that becomes conductive when the voltage obtained by the voltage dividing resistor is larger than a predetermined set voltage;
A detection resistor connected in series to the shunt regulator;
The overcurrent detection circuit according to claim 1, comprising:   The overcurrent detection circuit according to claim 2, further comprising a photocoupler that transmits a signal related to a voltage across the detection resistor to the primary side.

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